Laser repetition rate multiplier and flat-top beam profile generators using mirrors and/or prisms

ABSTRACT

A repetition rate (pulse) multiplier includes one or more beam splitters and prisms forming one or more ring cavities with different optical path lengths that delay parts of the energy of each pulse. A series of input laser pulses circulate in the ring cavities and part of the energy of each pulse leaves the system after traversing the shorter cavity path, while another part of the energy leaves the system after traversing the longer cavity path, and/or a combination of both cavity paths. By proper choice of the ring cavity optical path length, the repetition rate of an output series of laser pulses can be made to be a multiple of the input repetition rate. The relative energies of the output pulses can be controlled by choosing the transmission and reflection coefficients of the beam splitters. Some embodiments generate a time-averaged output beam profile that is substantially flat in one dimension.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 14/596,738, entitled “Laser Repetition Rate Multiplier And Flat-TopBeam Profile Generators Using Mirrors And/Or Prisms” which claimspriority to U.S. Provisional Patent Application Ser. No. 62/015,016,entitled “Laser Pulse Multiplication Using Prisms”, filed on Jun. 20,2014, and incorporated by reference herein, also claims priority to U.S.Provisional Patent Application Ser. No. 62/038,471 entitled “LaserRepetition Rate Multiplier and Flat-Top Beam Profile Generators”, filedon Aug. 18, 2014, and incorporated by reference herein.

This application is related to U.S. patent application Ser. No.13/487,075 entitled “Semiconductor Inspection And Metrology System UsingLaser Pulse Multiplier” and filed on Jun. 1, 2012 by Chuang et al., toU.S. Pat. No. 9,151,940 entitled “Semiconductor Inspection and MetrologySystem Using Laser Pulse Multiplier” and issued on Oct. 6, 2015 byChuang et al., to U.S. patent application Ser. No. 14/455,161, entitled“Split Gaussian Beams and Multi-Spot Flat-Top Illumination for SurfaceScanning Systems” and filed on Aug. 8, 2014 by Chuang et al. All ofthese applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to reducing the optical peak power oflaser pulses in the temporal domain and, optionally, to homogenizing thebeam power distribution in a spatial domain. This peak power reductionand homogenization system may use curved mirrors, beam splitters, waveplates, and prisms to generate an optimized pulse repetition-ratemultiplier with a flat-top spatial power distribution profile. Thepresent invention is particularly useful in semiconductor inspection andmetrology systems.

Related Art

The illumination needs for inspection and metrology are generally bestmet by continuous wave (CW) light sources. A CW light source has aconstant power level, which allows for images or data to be acquiredcontinuously. However, at many wavelengths of interest, particularlyultraviolet (UV) wavelengths, CW light sources of sufficient radiance(power per unit area per unit solid angle) are not available, areexpensive or are unreliable. If a pulse laser is the only available, orcost-effective, light source with sufficient time-averaged radiance atthe wavelength of interest, then using a laser with a high repetitionrate and wide pulse width is best. The higher the pulse repetition rate,the lower the instantaneous peak power per pulse for the sametime-averaged power level. The lower peak power of the laser pulsesresults in less damage to the optics and to the sample or wafer beingmeasured, as most damage mechanisms are non-linear and depend morestrongly on peak power rather than on average power.

In inspection and metrology applications, an additional advantage of anincreased repetition rate is that more pulses are collected per dataacquisition or per pixel leading to better averaging of thepulse-to-pulse variations and improved signal-to-noise ratios.Furthermore, for a rapidly moving sample, a higher pulse rate may leadto a better sampling of the sample position as a function of time, asthe distance moved between each pulse is smaller.

The repetition rate of a laser subsystem can be increased by improvingthe laser medium, the pump system, and/or its driving electronics.Unfortunately, modifying a UV laser that is already operating at apredetermined repetition rate can require a significant investment oftime and money to improve one or more of its constituent elements, andmay only improve the repetition rate by a small increment. Furthermoreincreasing the repetition rate of the fundamental laser in a UV laserreduces the peak power of the fundamental. This reduces the efficiencyof the frequency conversion (which is necessarily a non-linear process)and so makes it harder to generate high average UV power levels.

In many inspection applications, a flat or uniform, rather thanGaussian, illumination profile is desired. Spatially uniformillumination on the sample results in a more uniform signal-to-noiseratio across the illuminated area and a higher dynamic range comparedwith non-uniform illumination. Although incoherent light sources may beable to more readily generate uniform illumination than the Gaussianprofile of a laser, such light sources have much broader bandwidth(complicating the optical design because of chromatic aberration) andlower power density (reducing signal-to-noise ratios) than a laser canprovide. One known way to achieve an approximately flat profile from aGaussian laser beam is to crop off the Gaussian tails and only use thecentral region (close to peak) of the beam. This method is simple toapply; however, if a reasonably flat profile is required a largefraction of the laser power is cropped off and wasted. For example, ifthe maximum intensity variation in the illumination is required to beabout 10%, then about 65% of the power is wasted, and a 20% variationrequires wasting approximately 50% of the power.

Therefore, a need arises for a practical, inexpensive technique toimprove the repetition rate of a UV laser that operates on the output ofthe laser. Furthermore it would be advantageous if the optical subsystemthat increases the repetition rate can be compact so that it can readilybe incorporated into a system without taking up a lot of space. Stillfurthermore there is a need for a repetition rate multiplier that cangenerate an approximately flat output profile while adding no, or few,additional components to the repetition rate multiplier, thus savingspace and minimizing optical power losses.

SUMMARY OF THE INVENTION

A system for inspecting or measuring a sample is described. This systemincludes an illumination source, a device configured to perform lightdetection, optics configured to direct light from the illuminationsource to the sample and to direct light outputs, reflections, ortransmissions from the sample to a sensor. Notably, the illuminatorcomprises a pulsed laser emitting an ultra-violet (UV) wavelength (i.e.a wavelength shorter than about 400 nm) and a repetition rate multiplierthat multiplies the repetition rate of the pulses from the pulsed laser.The repetition rate multiplier increases the number of laser pulses perunit time and decreases the peak power of each laser pulse. Thedecreased peak power reduces or eliminates damage to the system opticsor the sample being inspected or measured, and allows use of a higheraverage laser power level for a given damage threshold, thus improvingthe signal-to-noise ratio and/or the speed of the inspection ormeasurement. Multiplying the repetition rate after generation of the UVharmonics maintains the efficiency of the UV harmonic conversion becausethe peak power of the laser pulses is not reduced in the harmonicconversion chain.

Inspection and measurement systems incorporating a repetition ratemultiplier as described herein are particularly useful at deep UV (DUV)wavelengths, i.e. wavelengths shorter than about 300 nm, and vacuum UV(VUV) wavelengths, i.e. wavelengths shorter than about 190 nm, as highpeak power levels at these wavelengths can quickly damage many differentkinds of materials.

The sample may be supported by a stage, which moves the sample relativeto the optics during the inspection or measurement.

The exemplary inspection or measurement system may include one or moreillumination paths that illuminate the sample from different angles ofincidence and/or different azimuth angles and/or with differentwavelengths and/or polarization states. The exemplary inspection ormeasurement system may include one or more collection paths that collectlight reflected or scattered by the sample in different directionsand/or are sensitive to different wavelengths and/or to differentpolarization states.

Inspection and measurement systems incorporating a repetition ratemultiplier may be further configured to generate a time-averagedspatially uniform beam profile (i.e. a flat-top profile). An inspectionor measurement system incorporating a multiplier and flat-top profilegenerators described herein provide two or more times multiplication ofthe laser repetition rate and a more uniform time-averaged beam profileusing a small number of optical components in a compact space. Theinspection and metrology systems described herein are capable of usinghigher average laser power enabling a higher throughput, better signalquality, and more efficient use of the laser energy.

Method and systems for multiplying the repetition rate of a pulsed laserare described. These methods and systems split an input laser pulse intomultiple pulses separated in time so as to multiply the laser repetitionrate by an integer such as 2, 3 or 4. An incoming pulse is split intotwo so that part of the pulse continues on, and part of the pulse entersa ring cavity. After traversing at least a section of the ring cavity,the pulse is split again and part of the pulse leaves the ring cavityand part continues on in the ring cavity. The repetition rate multipliermay be further configured to generate a time-averaged output profilethat is approximately flat in one dimension and substantially Gaussianin the perpendicular dimension. The repetition rate multiplier maycomprise flat mirrors, curved mirrors, polarized beam splitters,wave-plates, beam compensators and/or lenses.

In one exemplary embodiment, an input laser pulse is split into two by awave plate and a polarized beam splitter. One part of the input laserpulse is directed around a short ring cavity loop and the other part isdirected around a long ring cavity loop. On their way back to theinput/output coupler (which may comprise a polarized beam splitter), thepulses encounter another wave plate which determines the fraction of thepulse energy that leaves the cavities. The remaining fraction of thepulse energy traverses again the cavities.

In one exemplary embodiment the short and long cavity loop lengths arerespectively set to be ⅓ and ⅔ of the input laser pulse-to-pulse spatialseparation so that the output pulses will be delayed in time by ⅓, ⅔ oran integer multiple of ⅓ of the pulse-to-pulse period. These delayedpluses form a pulse train with a repetition rate that is three timesthat of the original input laser pulses. The orientation and retardanceof the two wave plates may be chosen such that the output pulses havesubstantially equal pulse-to-pulse energy.

In another exemplary embodiment, which can also triple the repetitionrate, two mirrors form a ring cavity and two beam splitters are placedin between. Whenever a pulse goes through a beam splitter, it splitsinto two pulses; one of the pulses goes straight through while the otheris deflected. With these two beam splitters, some pulses traverse alonger cavity loop while others traverse a shorter one. In one exemplaryembodiment, the shorter loop has a path length approximately equal toabout ⅓ of the original input pulse-to-pulse separation, and the longerloop path length is approximately ⅔ of the pulse-to-pulse separation. Inthis embodiment, the output pulses form a pulse train with a repetitionrate that is three times that of the original input pulses. With anappropriate choice of mirror curvature, mirror separation, andbeam-splitter reflectivities, the output pulses can have substantiallyequal pulse-to-pulse energy.

In one embodiment, two beam compensators comprising flat plates areplace in the cavity to substantially compensate for the shifts in thebeam path caused by the beam splitters so that the beams reflect fromthe mirrors in a pattern that is substantially symmetric about thecavity axis. In another embodiment, one (or both) beam compensators arereplaced by a prism (or prisms) that substantially compensates for theshift in beam path of one (or both) beam splitters. In yet anotherembodiment, no prism or beam compensator is used and the beam splittersare positioned in such a way that each compensates for the beam shiftcaused by the other.

In yet another exemplary embodiment, flat mirrors and prisms areinserted into the light path within the cavity to form a secondarycavity loop between the same pair of curved mirrors with a loop lengthabout half that of the primary cavity loop. If the primary cavity looplength is set to about half of the original input pulse-to-pulseseparation, the primary cavity loop can double the pulse repetitionrate. Pulses leaving the primary cavity loop enter the secondary cavityloop which has a length that is about half that of the primary cavityloop, thus doubling the pulse repetition rate again, resulting in anoutput pulse repetition rate that is four times that of the input laser.

Some embodiments use a prism such as an isosceles triangle prism or aDove prism to double the number of round trips that the beam makeswithin the cavity. The two cavity routes generate two parallel outputbeams. The deviation between these two beams can be chosen such thatthey overlap to form a time-averaged spatially approximately flat-topbeam profile.

In one embodiment, a 2× pulse multiplier scheme is used as a base forflat-top profile generation. In another embodiment, a 3× pulsemultiplier is used as a base for flat-top profile generation. In yetanother embodiment, the abovementioned 4× pulse multiplier scheme isused as a base for flat-top profile generation. This embodiment cangenerate four parallel beams with a predetermined power ratio betweenthem. By selecting the separations between the beams and the powerratios between them, a wider flatter time-averaged beam profile can beachieved. Any of these repetition-rate multipliers that generate anapproximately flat-top profile may comprise beam compensators and/orprisms.

In one embodiment, the ring cavity comprises right angle mirror pairs.In another embodiment, the ring cavity comprises prisms that utilizetotal internal reflection to achieve high reflectivity. With anappropriate prism design, a ring cavity using prisms can achieve lowlosses without using high reflectivity coatings. High reflectivitycoatings can easily be damaged by high intensity laser pulses,particularly at short wavelengths, so many of the methods and systemsdescribed herein can have a longer operating life and/or lowermaintenance costs compared with other ring cavities especially when usedfor multiplying the repetition rate of DUV and VUV lasers.

In one embodiment one or more prisms in the ring cavity are designed sothat the angle of incidence of the laser beam entering and exiting theprism is close to Brewster's angle and the laser beam is substantiallyP-polarized relative to the prism entrance and exit surfaces, so thatthe losses due to reflection are kept small without using anyanti-reflection (AR) coating. AR coatings can easily be damaged by highintensity laser pulses, particularly at short wavelengths, so thisembodiment can have a longer operating life and/or lower maintenancecosts compared with ring cavities using AR coatings especially when usedfor multiplying the repetition rate of DUV and VUV lasers.

In one embodiment, the Brewster cut of the prism(s) is oriented for thebeam polarization lies in the same ring-cavity plane, while in anotherembodiment the Brewster cut of the prism(s) is oriented for the beampolarization that is perpendicular to the ring-cavity plane.

In one embodiment, the beam is totally internally reflected twice in asingle prism of appropriate design. Such a prism can replace two foldingmirrors in a ring cavity, and so reduce the total number of componentsand simplify the process of aligning the ring cavity.

In one embodiment a right-angled prism is used in the ring cavity. Itreflects the beam twice which sends the beam back in the oppositedirection while also displacing it in space. This unique feature of theright-angled prism results in the flexibility to multiply the effectivecavity length by simply rotating the right-angle prism to a specificangle. For example, two ring cavities may be constructed with similarphysical lengths, but with one having an optical path length that is aninteger multiple of (such as twice) the optical path length of the otherso that the two ring cavities may be cascaded in order to multiply thepulse repetition rate by a larger factor then can be convenientlyachieved in a single ring cavity.

In a preferred embodiment, the fraction of the energy of each inputlaser pulse that is directed into the ring cavity is controlled byselecting the angle of incidence and polarization relative to a surfaceto achieve the desired reflection and transmission coefficients. Thishas the advantage of avoiding the need for any coating on the surfaceand so avoids the possibility of coating damage caused by the peak powerdensity of the laser pulses, which can be a problem particularly whenthe laser repetition rate multiplier is used with a deep UV or vacuum UVlaser with an average power of hundreds of mW or greater. Such lasersare increasingly needed in semiconductor inspection and metrologysystems in order to achieve the desired sensitivity and signal-to-noiseratio when inspecting or measuring features with dimensions of about 100nm or smaller.

In a preferred embodiment, one or more lenses are used within the ringcavity to re-image each laser pulse such that it retains approximatelythe same shape and size each time it traverses the cavity. Oneembodiment uses Brewster's angle lenses without coatings to refocus eachlaser pulse, thus avoiding the risk of coating damage.

In a preferred embodiment, two or more of the above described featuresare combined in one laser repetition rate multiplier. For example, inone preferred embodiment a laser repetition rate multiplier comprises aring cavity comprising three uncoated prisms wherein the laser beaminside the cavity is substantially p polarized relative to the surfacesof those prisms. Two of the prisms use total internal reflection tocirculate the laser beam efficiently within the ring cavity. A thirdprism has a surface that is oriented so that the laser beam isapproximately s polarized relative to that surface and the input pulsesare incident at an angle chosen so that desired fraction each inputpulse is directed into the ring cavity.

Wafer inspection systems, patterned wafer inspection systems, photomaskinspection systems, and metrology systems incorporating a laser pulsemultiplier are described. The compact size of the laser pulsemultipliers described herein makes them relatively easy to incorporateinto inspection and metrology systems. The use of uncoated optics in thelaser pulse multiplier allows those inspection and metrology systems tooperate with high powered deep UV lasers without performance degradationor maintenance issues due to coating damage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary inspection or measurement systemincorporating a pulsed laser and a laser pulse repetition ratemultiplier which may also be configured as a flat-top profile generator.

FIGS. 2A and 2B illustrates a dark-field inspection system thatincorporates a repetition rate multiplier which may also be configuredas a flat-top profile generator.

FIG. 3 illustrates an inspection system configured to detect particlesor defects on a sample using both normal and oblique illumination beamswhich can also benefit from incorporating a repetition rate multiplierwhich may also be configured as a flat-top profile generator.

FIG. 4 an exemplary catadioptric imaging system configured as aninspection system with bright-field and dark-field inspection modeswhich can advantageously incorporate a repetition rate multiplier whichmay also be configured as a flat-top profile generator.

FIG. 5 illustrates an exemplary repetition rate multiplier thatgenerates an output pulse train that has a pulse repetition rate that isthree times that of the input pulses.

FIG. 6 illustrates another exemplary repetition rate multiplierembodiment that also generates an output pulse train that has arepetition rate that is three times that of the input pulses.

FIGS. 7A and 7B illustrate exemplary repetition rate multipliers similarto the one shown in FIG. 6 except that no beam compensator is used.

FIG. 8 illustrates an exemplary repetition rate multiplier thatgenerates an output pulse train with a repetition rate that is fourtimes that of the input pulses.

FIG. 9 illustrates an exemplary 2× pulse repetition rate multiplier,which uses an isosceles triangle prism instead of a plate beamcompensator.

FIGS. 10A and 10B illustrate the features of an isosceles triangle prismincluding shifting the beam and reversing the spatial sequence of thebeams.

FIG. 11A illustrates an exemplary embodiment which generates a flat-topoutput beam profile by shifting the input beam illustrated in FIG. 9.

FIG. 11B illustrates another exemplary embodiment which generates aflat-top profile by shifting the prism towards the cavity axis.

FIG. 12 illustrates a flat-top profile formed by two partiallyoverlapped Gaussian beams.

FIG. 13 illustrates an exemplary flat-top profile generator based on the3× pulse repetition rate multiplier scheme shown in FIG. 6 where atleast one plate compensator is replaced by a prism.

FIG. 14A illustrates an exemplary flat-top profile generator based onthe 3× pulse repetition rate multiplier scheme shown in FIG. 7A.

FIG. 14B illustrates another exemplary flat-top profile generator basedon 3× pulse repetition rate multiplier scheme shown in FIG. 7B.

FIG. 15A illustrates another exemplary flat-top profile generator basedon the 4× pulse repetition rate multiplier scheme shown in FIG. 8.

FIG. 15B illustrates an exemplary flat-top profile generated by thedesign shown in FIG. 15A.

FIGS. 16A and 16B are front and side views showing a generator based ona 2× pulse repetition rate multiplier according to another embodiment.

FIGS. 16C and 16D are front and top views showing a generator based on a2×/4× pulse repetition rate multiplier according to another embodiment.

FIG. 17 illustrates an exemplary repetition rate multiplier with 2 rightangle prisms and a beam splitter.

FIG. 18 illustrates another exemplary repetition rate multiplier similarto Figure A except that one of the prisms is rotated 90° to double thedelay time of laser pulses for a given distance between the prisms.

FIGS. 19A, 19B and 19C are simplified diagrams illustrating how, inaccordance with one embodiment, the ring cavity length can be changed todifferent integer multiples of the physical cavity length by rotatingone of the right angle mirror pairs or prisms.

FIG. 20 illustrates one possible embodiment to generate a pulse trainwith a repetition rate that is 4 times the rate of the input pulse trainusing two cavities of similar external dimensions.

FIG. 21 illustrates one exemplary lens configuration used in a pulserepetition rate multiplier to maintain the laser beam quality of eachpulse as it travels around the ring cavity.

FIG. 22 illustrates a lens configuration similar to FIG. 21 whichretains the laser beam quality in a ring cavity that has one of theprisms rotated relative to the other.

FIG. 23 illustrates an alternative exemplary lens configuration used ina pulse repetition multiplier to retain the laser beam quality of eachpulse as it travels around the ring cavity.

FIG. 24 illustrates another exemplary lens configuration used in a pulserepetition rate multiplier to retain the laser beam quality usingcylindrical lenses with the laser incident on each lens at an angleapproximately equal to Brewster's angle.

FIGS. 25A and 25B illustrates two exemplary pulse repetition ratemultipliers with special prism designs that combine the beam splitterfunction into one of prisms. Both repetition rate multipliers can useBrewster angle prisms for recirculating the light in the cavity. Therepetition rate multiplier of FIG. 25B can use all uncoated opticsavoiding the possibility of coating damage by high intensity deep UVlaser pulses.

FIGS. 26A and 26B illustrate exemplary repetition rate multiplierssimilar to those of FIG. 25A but with one of the prisms rotated 90° todouble the pulse delay time.

FIGS. 27A and 27B illustrate exemplary repetition rate multipliers using3 prisms without coatings and no separate beam splitter. As for theother embodiments using all uncoated optics, this repetition ratemultiplier is particularly suitable for use in the deep UV because itavoids the possibility of coating damage by the laser pulses.

FIGS. 28, 28A, 28B and 28C illustrate details of the design of a firstprism of FIG. 27A.

FIGS. 29A and 29B are graphs illustrating Fresnel reflection as afunction of angle of incidence for both external and internalreflection. The angle of incidence used in one embodiment of the firstprism of FIG. 27A is shown.

FIGS. 30, 30A and 30B illustrate details of the design of a second prismof FIG. 27A.

FIGS. 31, 31A, 31B and 31C illustrate details of the design of a thirdprism of FIG. 27A.

FIGS. 32A and 32B illustrate an alternative exemplary repetition ratemultiplier using two prisms and a beam splitter, or one prism, onemirror and one beam splitter.

FIG. 33 illustrates an exemplary 4× repetition rate multipliercomprising two ring cavities similar to that shown in FIG. 32A where oneof the ring cavities has its prism rotated through 90° so as to doubleits effective cavity length.

DETAILED DESCRIPTION OF THE DRAWINGS

Improved illumination systems for semiconductor inspection andmeasurement systems are described herein. The following description ispresented to enable one of ordinary skill in the art to make and use theinvention as provided in the context of a particular application and itsrequirements. Various modifications to the described embodiments will beapparent to those with skill in the art, and the general principlesdefined herein may be applied to other embodiments. Therefore, thepresent invention is not intended to be limited to the particularembodiments shown and described, but is to be accorded the widest scopeconsistent with the principles and novel features herein disclosed.

FIG. 1 illustrates an exemplary inspection system 100 configured toinspect or measure a sample 108, such as a wafer, reticle, or photomask.Sample 108 is placed on a stage 112 to facilitate movement of differentregions of sample 108 underneath the optics. Stage 112 may comprise anX-Y stage or an R-θ stage. In some embodiments, stage 112 can adjust theheight of sample 108 during inspection to maintain focus. In otherembodiments, an objective lens 105 can be adjusted to maintain focus.

An illumination source 102 comprises one or more pulsed lasers and arepetition rate multiplier as described herein. Illumination source 102may emit DUV and/or VUV radiation. Optics 103, including an objectivelens 105, directs that radiation towards and focuses it on sample 108.Optics 103 may also comprise mirrors, lenses, and/or beam splitters (notshown in detail for simplicity). Light reflected or scattered fromsample 108 is collected, directed, and focused by optics 103 onto adetector 106, which is within a detector assembly 104.

Detector 106 may include a two-dimensional array sensor or aone-dimensional line sensor. In one embodiment, the output of detector106 is provided to a computing system 114, which analyzes the output.Computing system 114 is configured by program instructions 118, whichcan be stored on a carrier medium 116.

Illumination source 102 includes a pulsed laser 119 and a repetitionrate multiplier 120. In one embodiment, illumination source 102 mayfurther include a continuous source, such as an arc lamp, a laser-pumpedplasma light source, or a CW laser.

One embodiment of inspection system 100 illuminates a line on sample108, and collects scattered and/or reflected light in one or moredark-field and/or bright-field collection channels. In this embodiment,detector 106 may include a line sensor or an electron-bombarded linesensor. In this embodiment repetition rate multiplier 120 withinillumination source 102 may be configured to generate a flat-top profileso as to efficiently generate a substantially uniform line illumination.

Another embodiment of inspection system 100 illuminates multiple spotson sample 108, and collects scattered and/or reflected light in one ormore dark-field and/or bright-field collection channels. In thisembodiment, detector 106 may include a two-dimensional array sensor oran electron-bombarded two-dimensional array sensor.

Additional details of various embodiments of inspection system 100 aredescribed in U.S. Published Application 2013/0016346 entitled “Waferinspection system”, published on Jan. 17, 2013, U.S. Pat. No. 7,957,066entitled “Split Field Inspection System Using Small CatadioptricObjectives”, issued on Jun. 7, 2011, U.S. Pat. No. 7,345,825 entitled“Beam Delivery System For Laser Dark-field Illumination in aCatadioptric Optical System”, issued on Mar. 18, 2008, U.S. Pat. No.5,999,31, entitled “Ultra-Broadband UV Microscope Imaging System WithWide Range Zoom Capability”, issued on Dec. 7, 1999, and U.S. Pat. No.7,525,649 entitled “Surface Inspection System Using Laser LineIllumination With Two Dimensional Imaging”, issued on Apr. 28, 2009. Allof these patents are incorporated by reference herein.

FIGS. 2A and 2B illustrate aspects of a dark-field inspection system 200that incorporates a repetition rate multiplier and/or a repetition ratemultiplication method described herein in accordance with otherexemplary embodiments of the present invention. In FIG. 2A, illuminationoptics 201 comprises a laser system 220, which generates light 202 thatis focused by a mirror or lens 203 into a line 205 on surface of a waferor photomask (sample) 211 being inspected. Collection optics 210 directslight scattered from line 205 to a sensor 215 using lenses and/ormirrors 212 and 213. An optical axis 214 of collection optics 210 is notin the illumination plane of line 205. In some embodiments, optical axis214 is approximately perpendicular to line 205. Sensor 215 comprises anarray sensor, such as a linear array sensor. Laser system 220incorporates one or more of the repetition rate multipliers and/orrepetition rate multiplication methods described herein. Laser system220 may be configured to efficiently generate a flat-top profile inaccordance with an embodiment of the present invention so that thetime-averaged light intensity along line 205 may be substantiallyuniform.

FIG. 2B illustrates one embodiment of multiple dark-field collectionsystems 231, 232 and 233, each collection system substantially similarto collection optics 210 of FIG. 2A. Collection systems 231, 232 and 233may be used in combination with illumination optics substantiallysimilar to illumination optics 201 of FIG. 2A. Sample 211 is supportedon stage 221, which moves the areas to be inspected underneath theoptics. Stage 221 may comprise an X-Y stage or an R-θ stage, whichpreferably moves substantially continuously during the inspection toinspect large areas of the sample with minimal dead time.

More details of inspection systems in accordance with the embodimentsillustrated in FIGS. 2A and 2B are described in U.S. Pat. No. 7,525,649,entitled “Surface inspection system using laser line illumination withtwo dimensional imaging”, issued on Apr. 28, 2009, and U.S. Pat. No.6,608,676, entitled “System for detecting anomalies and/or features of asurface”, issued on Aug. 19, 2003. Both of these patents areincorporated by reference herein.

FIG. 3 illustrates an inspection system 300 configured to detectparticles or defects on a sample using both normal and obliqueillumination beams. In this configuration, a laser system 330 provides alaser beam 301. Laser system 330 comprises a pulsed laser and arepetition rate multiplier as described herein. A lens 302 focuses beam301 through a spatial filter 303. Lens 304 collimates the beam andconveys it to a polarizing beam splitter 305. Beam splitter 305 passes afirst polarized component to the normal illumination channel and asecond polarized component to the oblique illumination channel, wherethe first and second components are orthogonal. In a normal illuminationchannel 306, the first polarized component is focused by optics 307 andreflected by a mirror 308 towards a surface of a sample 309. Theradiation scattered by sample 309 (such as a wafer or photomask) iscollected and focused by a paraboloidal mirror 310 to a sensor 311.

In an oblique illumination channel 312, the second polarized componentis reflected by a beam splitter 305 to a mirror 313 which reflects suchbeam through a half-wave plate 314 and focused by optics 315 to sample309. Radiation originating from the oblique illumination beam in obliquechannel 312 and scattered by sample 309 is collected by paraboloidalmirror 310 and focused to sensor 311. Sensor 311 and the illuminatedarea (from the normal and oblique illumination channels on sample 309)are preferably at the foci of paraboloidal mirror 310.

Paraboloidal mirror 310 collimates the scattered radiation from sample309 into a collimated beam 316. Collimated beam 316 is then focused byan objective 317 and through an analyzer 318 to sensor 311. Note thatcurved mirrored surfaces having shapes other than paraboloidal shapesmay also be used. An instrument 320 can provide relative motion betweenthe beams and sample 309 so that spots are scanned across the surface ofsample 309.

U.S. Pat. No. 6,201,601, entitled “Sample Inspection System”, issued onMar. 13, 2001, and U.S. Published Application 2013/0016346 entitled“Wafer Inspection System”, published on Jan. 17, 2013 by Romanovsky etal., both of which are incorporated by reference herein, describeinspection system 300 in further detail.

FIG. 4 illustrates an exemplary catadioptric imaging system 400configured as an inspection system with bright-field and dark-fieldinspection modes. System 400 incorporates two illuminations sources: alaser 401 and a broad-band light illumination module 420. Laser 401comprises a pulsed laser and a repetition rate multiplier as describedherein. In preferred embodiments, laser 401 comprises a DUV or VUVlaser, a pulse repetition rate multiplier and/or a flat-top profilegenerator as described herein.

In a dark-field mode, adaptation optics 402 control the laserillumination beam size and profile on the surface being inspected. Amechanical housing 404 includes an aperture and window 403, and a prism405 to redirect the laser along the optical axis at normal incidence tothe surface of a sample 408. A prism 405 also directs the specularreflection from surface features of sample 408 out of an objective 406.Objective 406 collects light scattered by sample 408 and focuses it on asensor 409. Lenses for objective 406 can be provided in the general formof a catadioptric objective 412, a focusing lens group 413, and a tubelens section 414, which may, optionally, include a zoom capability.

In a bright-field mode, a broad-band illumination module 420 directsbroad-band light to a beam splitter 410, which reflects that lighttowards focusing lens group 413 and catadioptric objective 412.Catadioptric objective 412 illuminates sample 408 with the broadbandlight. Light that is reflected or scattered from the sample is collectedby objective 406 and focused on sensor 409. Broad-band illuminationmodule 420 comprises, for example, a laser-pumped plasma light source oran arc lamp. Broad-band illumination module 420 may also include anauto-focus system to provide a signal to control the height of sample408 relative to catadioptric objective 412.

U.S. Pat. No. 7,245,825 entitled “Beam Delivery System For LaserDark-Field Illumination in a Catadioptric optical system”, issued onMar. 18, 2008 and incorporated by reference herein, describes system 400in further detail.

FIG. 5 illustrates an exemplary pulse repetition rate multiplier 120Aconfigured to receive input laser pulses (input), and to generate apulse train (output) with a repetition rate that is three times that ofthe input laser pulses. Similar to the scheme described in the abovecited Ser. No. 13/487,075 U.S. patent application, a polarized beamsplitter (PBS) A01 serves as the input and output coupler of a ringcavity. The input laser is p polarized relative to PBS A01. PBS A01 isdesigned and oriented so as to receive the input laser pulse, to pass ppolarization and reflect s polarization. Two additional PBS (A02 andA03) and three folding mirrors (A04, A05, A06) form a dual cavity. Thedual cavity also includes two half-wave plates: one is placed betweenA01 and A02, while another one is placed between A03 and A06. When alaser pulse enters the dual cavity through PBS A01, it will be dividedinto two pulses by PBS A02. One part of the pulse is reflected from PBSA02 to PBS A03, directed from PBS A03 to mirror A06, then reflected frommirror A06 back to PBS A01 (shown as loop A). The other part of thepulse is transmitted through PBS A02 to mirror A04, reflected frommirror A04 to mirror A05, reflected from mirror A05 through A03 tomirror A06, then reflected from mirror A06 back to PBS A01 (shown asloop B). Repetition rate multiplier 120A also includes a first waveplate A07 for changing a polarization of the input laser pulse portionpassed from PBS A01 to PBS A02, and a second wave plate A08 for changinga polarization of input laser pulse portions passed from PBS A03 tomirror A06. The laser pulse energy distribution between loop A and loopB can be controlled by the angle of the principle axis of half-waveplate A07 relative to the polarization of the input laser pulse. Byselecting the angle of the principle axis of half-wave plate A08, onecan control the ratio of the pulse energy recycled into the dual cavityto the energy that exits the dual cavity through PBS A01.

In one preferred embodiment, the optical path length of loop A is set toapproximately one-third of the pulse-to-pulse distance of input laserpulses, and path length of loop B is set to approximately twice that ofloop A. This results in output pulses at approximately one third and twothirds of the time interval between input laser pulses and approximatelycoincident with the input pulses, thus tripling the repetition rate ofthe laser. In this embodiment, preferably the angles α and β of theprincipal axes of wave plates A07 and A08 respectively are set to beeither approximately α=29° and β=16°, respectively, or approximatelyα=16° and β=29°, respectively, so as to produce approximately equalenergy in each output pulse. If small differences (such as a fewpercent) in the energy of each pulse are acceptable in a specificapplication, then angles that differ from these values by one or twodegrees may be acceptable. Lenses (not shown) may be incorporated intothe dual cavity and/or one of more of mirrors A04, A05 and A06 may becurved such that the Gaussian beam waist and size of each pulse isre-imaged to the same condition when it returns to the same position.

Repetition rates other than three are possible with this dual cavity.For example Loop A may be set to have an optical path length equal toapproximately one quarter of the separation of input pulses, and Loop Bmay be set to be approximately twice the length of Loop A. This wouldresult in quadrupling the repetition rate of the input laser pulses.However such a scheme cannot generate equal output pulse energies and sowould only be useful when equal output pulse energies are not required.

FIG. 6 illustrates another pulse repetition rate multiplier 120B thatcan triple the repetition rate. Similar to the Herriott cell schemedescribed in the above cited '940 patent, this pulse repetition ratetripler comprises an optical cavity formed by a pair of curved mirrors(B01 and B02). Curved mirrors B01 and B02 are preferably sphericalmirrors. Pulse repetition rate tripler 120B further comprises two beamsplitters (B03, B04) and two beam compensators (B05, B06). The radii ofcurvature of the two curved mirrors B01 and B02 should, preferably, bothbe substantially equal to the distance between them (i.e. the cavityshould be confocal).

Laser input pulses (input) arrive at beam splitter B03. Part of theenergy of each pulse is reflected from beam splitter B03 to point B07 oncurved mirror B01, then to point B08 on curved mirror B02, through beamsplitter B04 to point B09 on curved mirror B01, then to point B10 oncurved mirror B02, and back to beam splitter B03. The other part of theenergy of each pulse is transmitted through beam splitter B03, to beamsplitter B04 where it is reflected to point B09 on curved mirror B01,then to point B10 on curved mirror B02, and back to beam splitter B03.In preferred embodiments, the optical path length of the shorter loop(B03-B04-B09-B10-B03) is approximately half of that of the longer one(B03-B07-B08-B04-B09-B10-B03). When the distance between the two curvedmirrors B01 and B02 is approximately one-sixth of the originalpulse-to-pulse spatial separation of input laser beam, the output pulsetrain will have triple the repetition rate of the input pulses. Beamcompensators B05 and B06 have optical thicknesses and orientationschosen so as to substantially compensate for the displacement of thelaser beam within the cavity caused by beam splitters B03 and B04respectively.

Similar to the output of the 2× repetition rate multiplier described inthe above-cited '940 patent and illustrated in FIGS. 2A and 2B of thatapplication, the output of pulse repetition rate tripler 120B consistsof a series of pulse trains, each pulse train comprising a series ofpulses that have traversed one or both of the cavities one or more time.Pulse repetition rate tripler 120B has three output pulse trains perinput pulse compared with two output pulses per input pulse for the 2×repetition rate multiplier. In a preferred embodiment of the pulserepetition rate tripler, the total energies in each output pulse trainare made approximately equal to one another by setting thereflectivities of beam splitters B03 and B04 to be approximately equalto

${\frac{1}{10}\left( {5 - \sqrt{5}} \right)\mspace{20mu}{and}\mspace{14mu}\frac{1}{10}\left( {5 + \sqrt{5}} \right)},$i.e. approximately 0.28 and approximately 0.72. Note that either B03 canhave a reflectivity of approximately 0.28 and B04 can have areflectivity of approximately 0.72, or B04 can have a reflectivity ofapproximately 0.28 and B03 can have a reflectivity of approximately0.72. Both configurations produce substantially equal output pulseenergies. Since pulse-to-pulse energy variations of a few percent may beacceptable in many inspection applications, the beam splitterreflectivities may be chosen to have values that differ by a few percentfrom 0.28 and 0.72. As one skilled in the relevant arts understands, thereflectivity of a beam splitter can be controlled by the selection ofthe beam splitter material, the thickness(es) and material(s) of anylayer or layers coated on the surface, and the angle of incidence on thebeam splitter.

FIG. 7A illustrates a repetition rate multiplier 120B-1 according toanother embodiment that utilizes curved mirrors (B01 and B02) and beamsplitters in a manner similar to that illustrated in FIG. 6, but differsin that the two beam compensators (B05 and B06, see FIG. 6) are removed.FIG. 7A shows that with proper adjustment of the positions of beamsplitters B03-1 and B04, the beam displacement caused by one of the beamsplitters can be compensated by the other beam splitter and vice versa.Preferably the two beam splitters B04 and B03-1 have substantially equaloptical thicknesses. FIG. 7B illustrates a repetition rate multiplier120B-2 according to another embodiment where beam splitter B03-2 isplaced with its coated side flipped to the other direction with respectto beam splitter B03-1 of FIG. 7A. In either embodiment, a closed loopcan exist and no beam compensator is needed.

FIG. 8 illustrates another repetition rate multiplier 120D which canquadruple the repetition rate. Repetition rate multiplier 120D comprisesan optical cavity formed by two curved mirrors B01 and B02 similar tothose described above with reference to FIG. 6, two beam splitters D01and D06, and two fold mirrors D05 and D07. The input laser repetitionrate gets doubled first in a manner similar to that described in theabove cited '940 patent, by using one beam splitter D01 (preferably witha reflectivity of approximately ⅔), and one beam compensator or prismD02 (the primary cavity loop). After that, the output beam D03 getsdiverted back to the cavity by right-angled prism D04 and a mirror D05.The beam then reaches another beam splitter D06 (preferably with areflectivity of approximately ⅓) along the path shown as a dotted line,and starts the secondary cavity loop (dashed lines) from D06, tospherical mirror B01, spherical mirror B02, then another flat mirrorD07, and back to beam splitter D06. The length of this secondary cavitypath loop is approximately half that of the first loop, thus it doublesthe repetition rate a second time and makes an output pulse train atfour times the initial input pulse repetition rate.

A special feature of this scheme is that this secondary cavity loop,which further multiplies the repetition rate a second time, utilizessame set of curved mirrors (B01 and B02) as the first cavity loop. Inaddition, the flat mirrors D05 and D07 can be combined into one opticalelement with high reflectivity (HR) coatings on both sides. Thesefeatures give rise to a more compact footprint as compared with a setupcomprising two individual 2× pulse multipliers cascaded together. Notethat, though convenient, it is not required to combine mirrors D07 andD05, and the beam D03 may be directed along a different path from thatshown to arrive at beam splitter D06. Alternative layouts are possibleand are within the scope of this embodiment.

FIG. 9 illustrates another repetition rate multiplier 120E which doublesthe repetition rate as discussed previously. Repetition rate multiplier120E comprises an optical cavity formed by curved mirrors B01 and B02and a beam splitter B03 in a manner similar to that described above withreference to FIG. 6. An isosceles triangle prism E01 is used here toreplace the compensator B05 from the previous embodiment.

FIGS. 10A and 10B illustrate useful features of isosceles triangle prismE01: (1) it shifts the beam and the amount of shift is adjustable bylaterally moving the prism, and (2) if multiple beams go into the prismin parallel, the spatial sequence of output beams will be reversed. Thisisosceles triangle prism can also be implemented as an isoscelestrapezoid or as a Dove prism.

FIG. 11A and FIG. 11B illustrate two similar flat-top beam generators102E-1 and 102E-2 according to exemplary embodiments that respectivelyutilize repetition rate multipliers 120E-1 and 120E-2 to split the inputlaser pulses (input) received from lasers 119E-1 and 119E-2 into twolaterally displaced (separate round-trip optical) output beam paths,thereby creating output laser pulses (output) having time-averagedflat-top beam profiles with a pulse repetition rate that is double (twotimes) the pulse repetition rate of the input laser pulses. Repetitionrate multipliers 120E-1 and 120E-2 include two spherical/cavity (curved)mirrors B01 and B02, a beam splitter B03, and isosceles triangle prismE01 arranged in a manner similar to the embodiment described above withreference to FIG. 9. FIG. 11A shows the nominal optical path of FIG. 9as a dotted line. In the embodiment shown in FIG. 11A, the beam in thecavity is shifted to route E02 by a small displacement of the input beam(e.g., by displacing laser 119E-1 in the direction indicated by thethick arrow at the bottom of FIG. 11A). After a laser pulse hascompleted one trip around the cavity on path E02, prism E01 laterallydisplaces the laser pulse to a path E03 on the opposite side of thenominal route. A pulse on route E03 gets switched to route E02 againwhen it arrives back at prism E01. Therefore, the laser pulses willalternate between routes E02 and E03.

Each time a laser pulse encounters beam splitter B03, part of the energyof the pulse gets reflected and exits the system. A pulse traveling onroute E02 generates a pulse on exit route E04, and a pulse on route E03generates a pulse on exit route E05. With this setup, one Gaussian beamsplits into two spatially. By controlling the separation between E04 andE05, the degree of overlap between these two laser beams can becontrolled. In a preferred embodiment the output beam profile possess anapproximately flat-top time-averaged intensity, as illustrated in FIG.12. An approximately flat top output beam profile can be created bydisplacing one Gaussian with respect to the other by approximately 0.5times the beam waist radius (i.e. the radius at which the beam powerdensity is 1/e² of its peak value, or equivalently the radius at whichbeam amplitude is 1/e of its peak value). This flat-top profile is verydesirable in many applications which a homogenized spatial powerdistribution is required. Note that, because laser pulses on paths E04and E05 leave the cavity at times separated by much longer than aduration of an individual pulse (such as separated in time byapproximately half the time interval between input laser pulses), thereis no interference of one pulse with another resulting in the desiredrelatively flat-top profile. Interference between the two displacedGaussians, which could occur without a long enough time delay betweenthe pulses, could cause a non-flat top of the profile.

FIG. 11B illustrates flat-top beam generator 102E-2 with repetition ratemultiplier 120E-2 configured according to another exemplary embodimentto generate laser pulses (output) having a flat-top beam profile.Instead of offsetting the input beam by shifting the position of laser119E-2, prism E01 is shifted toward the cavity axis (as indicated by thethick arrow at the bottom of the figure) and thus diverts pulses fromnominal route E06 (solid line) to new route E07 (dashed line). Eachpulse will then go back and forth (i.e., laterally displace) betweenroutes E06 and E07 every time it travels through prism E01 after acavity round trip. Similar to the embodiment in FIG. 11A, two outputbeams will be generated when the pulses go through beam splitter B03under two different routes and a time-averaged flat-top beam profile canbe formed by proper adjustment of the beam separation.

The above-mentioned repetition rate multipliers 120E-1 and 120E-2, whichfacilitate the flat-top schemes shown in FIG. 11A and FIG. 11B, arebased on a scheme that doubles the repetition rate (e.g., thearrangement described above with reference to FIG. 9). Therefore, it hasthe advantage that one optical cavity not only spreads out the laserpulse energy distribution in the time domain but also homogenizes theenergy distribution in the spatial domain.

FIG. 13 illustrates another flat-top beam generator 102F according toanother embodiment that utilizes repetition rate multiplier 120F toreceive input laser pulses generated by a laser 119F, and to generateoutput laser pulses F03 and F04 having a time-averaged flat-top beamprofile using a 3× pulse repetition rate multiplying system similar tothat described above and illustrated in FIG. 6. In this embodiment beamsplitter B04 is displaced downward from its nominal position (comparewith FIG. 6) and an isosceles or Dove prism F06 is used in place of beamcompensator B06 to laterally displace the beams in the manner describedabove, thus generating two beam paths. A laser pulse switches betweenouter route F01 (solid line) and inner route F02 (dashed line) everytime when the pulse goes through the path between beam splitter B03 andB04 (perpendicular to cavity axis) and when it passes prism F06. When alaser pulse encounters beam splitter B04, part of the energy of thepulse will go out of the system. In a preferred embodiment, the beamsplitter reflectivities are selected such that the average output powerof F03 is approximately equal to the average output power of F04. In onepreferred embodiment, where beam splitter losses are minimal, the beamsplitter reflectivities are chosen so that approximately

${R_{B\; 04} = \frac{R_{B\; 03}}{{4R_{B\; 03}} - 1}},$where R_(B03) and R_(B04) are the reflectivities of beam splitter B03and B04 respectively. Preferably the thicknesses of both beam splittersand the beam compensator are all equal so that it is straightforward toalign the optics to achieve two closed loops within the optical cavity.

Alternatively, FIGS. 14A and 14B illustrate exemplary flat-top beamgenerators 102G-1 and 102G-2 according other embodiments that utilizerepetition rate multipliers 120G-1 and 120G-2, respectively, havingconfigurations similar to FIG. 13 but without using any beam compensatoror prism, which receive input laser pulses generated by lasers 119G-1and 119G-2, respectively, and to generate output laser pulses (output)having flat-top beam profiles. With proper arrangement of the beamsplitter positions (e.g., by moving beam splitter B04-1 to the right asindicated in FIG. 14A, or by moving beam splitter B04-2 to the left asindicated in FIG. 14B), the generation of flat-top beam profiles withoutusing any compensator or prism is possible in 3× multiplier-basedscheme. In addition, the coatings of beam splitters can be either facingto different directions (FIG. 14A) or facing toward the same direction(FIG. 14B). You can also view the embodiments in FIGS. 14A and 14B asderivations of the arrangements shown in FIGS. 7A and 7B with one of thebeam splitters offset in location, which causes the beam to be splitinto two and, hence with the appropriate offset, generate atime-averaged flat-top output profile.

FIG. 15A illustrates another exemplary flat-top beam generator 102H thatutilizes a 4× repetition rate multiplier 120H having a scheme similar tothat shown in FIG. 8 (referred to as the ‘nominal’ here). Thisembodiment can generate a broader, flatter output profile than the 3×repetition rate multipliers. By utilizing the mechanism illustrated inFIG. 11B, shifting the prism D02 downward induces the first-stage outputbeam D03 to split into two (D03 and H01). Further, moving the prism D04toward left side shifts the split beams D03 and H01 to one side of thenominal path (dashed line). These two beams get multiplied to the otherside of the nominal after entering the second cavity formed by beamsplitter D06, mirror B01, mirror B02, and mirror D07. Therefore, therewill be four beams (H02, H03, H04, H05) leaving the system through beamsplitter D06. Note that the dotted lines in the figure are the nominalbeam paths which only exist when the right-angled prism has not shifted,and they are just for reference here. No beam actually goes through thisnominal route in this scenario.

In order to generate a flat-top beam ensemble, three parameters need tobe arranged in an appropriate relation. By proper adjusting the shiftingdistance of prism D02, one can tune the space (a) between D03 and H01,and hence the space between H02 and H03, and the space between H04 andH05 as well. By adjusting the displacement of prism D04, one can tunethe space between H03 and H04 (b). Lastly, one can choose the beamsplitter D01 reflectivity such that beams D03 and H01 possess differentpowers with a desired ratio.

FIG. 15B illustrates an exemplary output beam profile generated fromthis setup of FIG. 15A. In this exemplary embodiment, for an inputGaussian beam radius w (1/e² definition), a flat-top output profile isgenerated by a˜0.9w, b˜0.86w, the reflectivity of beam splitter D01R_(D01)˜0.65, and the reflectivity of beam splitter D06 R_(D06)˜0.33.This example generates a time-averaged output profile with asubstantially flat top of width about 2.3w. Other combinations may alsowork depending on how flat a profile is required.

In the above cited Ser. No. 13/487,075 application and '940 patent whichare incorporated by reference herein, alternative embodiments of laserpulse repetition rate multipliers are described. These applicationsexplain how the repetition rate of a pulsed laser may be doubled using aring cavity of appropriate length by using a beam splitter to directapproximately ⅔ of the energy of each input pulse into the cavity whiledirecting approximately ⅓ of the energy of each pulse to the output.With a cavity optical path length corresponding to approximately halfthe time interval between the input laser pulses, the output pulsetrains form envelopes of substantially similar energy which repeat at arepetition rate that is twice that of the original laser pulses. The'940 patent also describes how to adjust the transmission andreflectivity of the beam splitter so as to maintain substantially equaloutput pulse energies to compensate for losses in the beam splitter andring cavity. Any of the principles described in the Ser. No. 13/487,075application and '940 patent may be applied as appropriate to the variousembodiments of pulse repetition rate multipliers described herein.

A detailed description of one or more embodiments of the invention isprovided above along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment.

For example, in one embodiment, the optical components can be coatedwith appropriate coatings for the laser wavelength. Each surface of anytransmissive elements, such as wave-plates, can also have ananti-reflection coating that minimizes the amount of laser energyreflected at each surface. Mirrors can be coated with coatings designedto maximize the reflection and minimize scattering at the laserwavelength.

In another example, in one embodiment, the Herriott-cell-like cavity mayhave a different shape or a different number of mirrors from theexamples given above.

Although the above illustrated embodiments are drawn in one plane,alternative embodiments can place one of the cavity loops, such as thesecondary cavity loop in FIG. 8 or FIG. 15A, in a plane approximatelyperpendicular, or rotated relative, to the plane of another cavity loop,such as the primary cavity loop, while still using the same set ofmirrors. For example, FIGS. 16A and 16B are front and side views showinga 2× pulse repetition rate multiplier arrangement in which input pulsesand output pulses are directed in a plane perpendicular to the plane ofthe cavity loop formed by beam splitter D01, beam compensator or prismD02, and curved mirrors B01 and B02. FIGS. 16C and 16D are front and topviews showing a 4× arrangement in which one or more mirrors or prismscan be used outside of the cavity to direct or deflect the light fromprism D04 to mirror D05 lying in the plane of the secondary loop. Notein FIG. 16D, the input and output beams, the plane of the 2× cavityloop, beam splitter D01 and beam compensator or prism D02 are shown asseen from above by dashed lines for reference. The optical components ofone cavity loop are positioned so that they do not intercept the othercavity loop. An advantage of placing different cavity loops in differentplanes is that it is possible for each of the cavity loops to bereflected from the curved mirrors, such as mirrors B01 and B02, atsubstantially similar distances from the centers of those mirrors (asshown in FIG. 16C, where the dashed lines show the input and outputlight paths and the plane of the 2× loop as viewed from the front),allowing the cavity loops to be in focus at the same time and sominimize aberrations of the laser beam as it traverses the cavity loopsmultiple times. By orienting and displacing prisms D02 and D04 inappropriate directions in a manner similar to that illustrated in FIG.15A, the 4× laser pulse repetition rate multiplier shown in FIGS. 16Cand 16D can generate an output with a time-averaged substantiallyflat-top profile similar to that shown in FIG. 15A.

FIG. 17 illustrates an exemplary laser pulse repetition rate multiplier120I configured to generate a pulse train with a repetition rate that istwice that of the rate of the input laser pulses, which are generated bya laser (not shown) in the manner described above. Similar to theconcept described in the above cited '940 patent, a beamsplitter (I01)is placed in a ring cavity with an optical path length that isapproximately equivalent to half the time interval between twoconsecutive input laser pulses. The ring cavity comprises tworight-angle reflective-pair optical elements such as prisms (I02 andI03) which reflect the laser pulses by total internal reflection (TIR).Right-angle reflective-pair optical elements I02 and I03, have severaladvantages over mirrors when used in the ring cavity of laser repetitionrate multiplier 120I. One disadvantage of mirrors is that the mirrorsneed high reflectivity coatings in order to minimize losses as the laserpulses circulate in the ring cavity. High reflectivity coatings can bedamaged by the peak power of the laser pulses, particularly for deep UVlasers with powers of hundreds of mW or higher. The use of TIR insteadof high reflectivity coatings eliminates the risk of those coatingsbeing damaged under long-term operation with high laser power. A secondadvantage of the use of prisms rather than mirrors is that the fourmirrors needed to form two mirror-based right-angle reflective-pairelements are replaced by two prisms, reducing the number of opticalcomponents. A third advantage of the use of prisms is that the rightangle between two TIR surfaces of one prism is fixed and can bemanufactured with high precision. The tight angular tolerances of theprisms and the reduced number of optical components simplify thealignment of the ring cavity of FIG. 17.

As explained in the '940 patent, in a laser pulse repetition ratedoubler, if it is desired that the each output pulse be of substantiallyequivalent total energy, then the beam splitter I01 should be designedto reflect a substantially ⅔ (second) fraction of the energy of eachinput laser pulse into the ring cavity, and to transmit a substantially⅓ (first) fraction of each input laser pulse such that the ⅓ fractionexits repetition rate multiplier 120I at a first time, and such that the⅔ fraction exits repetition rate multiplier 120I at a second time afterbeing reflected between reflective elements I02 and I03. This can beachieved, for example, by the use of an appropriate coating on beamsplitter I01. Note that if substantially equal output pulse energies arenot required, then beam splitter I01 may be designed to reflect somefraction of each laser pulse other than ⅔. For example, when used in aninspection system, it may be desirable that each output pulse havesubstantially equal peak power to allow operation close to the damagethreshold of the article being inspected. Substantially equal peakpowers of the output pulses can be achieved using a beam splitter thanreflects about 62% of each laser pulse into the ring cavity.

As explained in the '940 patent, the optical path length of the ringcavity may be set to be slightly longer than, or slightly shorter than,the distance equivalent to half the time interval between input laserpulses so as to broaden the output pulses and reduce the peak power ofeach output pulse and so reduce the possibility of damage to down-streamoptics or to an article being inspected or measured by a systemincorporating a laser repetition rate multiplier.

For example if the input laser pulses have a repetition rate of 120 MHz,then an ring cavity optical path length of about 1.249 m would result inthe repetition rate being doubled with the output pulses beingapproximately equally spaced in time. To achieve this ring cavityoptical path length, the physical distance between the prisms would needto be about 0.625 m. As understood by one skilled in the appropriatearts, since the laser pulses travel a short distance inside each prismI01 and I02 and the refractive index of the prism material is greaterthan 1, the optical path length within the prism is a little longer thanthe physical distance traveled by the laser pulses inside the prism. Anappropriate small adjustment can be made to the physical distancebetween the prisms in order to compensate for this in order to achievethe desired ring cavity optical path length. If it is desired that theoutput pulses be broader than the input pulses in order to reduce thepeak power of each pulse, then the ring cavity optical path length canbe set to be a little longer or a little shorter than 1.249 m, such as aring cavity optical path length of 1.25 m so that a pulse that hastraveled twice around the ring cavity arrives about 6 ps (picoseconds)later than the next input pulse.

In a preferred embodiment, the ring cavity of repetition rate multiplier120I preferably includes an optical plate I04 to substantiallycompensate for the offset in the laser beam position caused by the laserpulses passing through beam splitter I01. Optical plate I04 shouldpreferably have an optical thickness substantially equal to the opticalthickness of beam splitter I01. Optical plate I04 should preferably becoated with an anti-reflection coating so as to minimize the reflectionof laser light from its surfaces. If optical plate I04 is placed in thesame arm of the ring cavity (as shown), then preferably optical plateI04 should be oriented at angle to the input laser beam (pulses) that issubstantially a mirror image of the angle to laser beam of the beamsplitter I01 so as to substantially compensate for the beam displacementcaused by the beam splitter I01. If the optical plate I04 is placed inthe other arm of the ring cavity (not shown), then it should preferablybe oriented substantially parallel to the beam splitter I01.

FIG. 18 illustrates another laser pulse repetition rate multiplier 120Jsimilar to that of FIG. 17 but with one of its right angle prisms (J02)rotated 90° relative to the other right angle prism (J03). Compared withthe configuration of FIG. 17, laser pulse repetition rate multiplier120J of FIG. 18 can achieve the same ring cavity optical path lengthwith substantially half the physical separation between the two prisms,thus resulting in a more compact laser pulse repetition rate multiplier.Beam splitter J01 and optical plate J04 perform the same functions asbeam splitter I01 and optical plate I04 of FIG. 17. Note that opticalplate J04 can be placed in any one of the four arms of the ring cavitywith an appropriate choice of orientation of the plate.

FIGS. 19A, 19B and 19C illustrate exemplary embodiments in which thering cavity optical path length is made substantially equal to double(FIG. 19A), quadruple (FIG. 19B), six times (FIG. 19C) or other eveninteger multiple of the physical separation of the prisms by rotatingone of the right-angle prisms (or equivalently right-angle mirror pairs)as shown to an appropriate angle with respect to the other prism/mirrorpair. The number of loops around the ring cavity which each pulse musttake before returning to its original launch position (for example thebeam splitter, which is not shown in FIGS. 19A-19C) is m=180°/θ, where θis the relative angle in degrees between the two prisms (or mirrorpairs). Referring to FIG. 19A, when θ=0, or equivalently 180°, m=1 andthe beam stays in the one plane and makes only one loop around the ringcavity before returning to its original position. Referring to FIG. 19B,when θ=90°, m=2 and each laser pulse makes two loops around the ringcavity in order to return to its original position. Referring to FIG.19C, when θ=60°, m=3 and the beam goes back to original launchingposition after three loops around the ring cavity. The polarization isalways preserved when the beam returns to its original position.

A key benefit of this design is maintaining substantially the samecavity footprint while changing the ring cavity optical path length bysimply rotating one of the prisms.

FIG. 20 illustrates another repetition rate multiplier 120K according toan exemplary embodiment in which two ring cavities of different opticalpath lengths are cascaded to quadruple the repetition rate of a pulsedlaser. The two ring cavities have substantially similar externaldimensions in spite of one ring cavity having twice the optical pathlength of the other. The first ring cavity comprises beam splitter K01,prisms K03 and K02, and optical plate K04. The first cavity doubles therepetition rate by using two loops around the ring cavity to achieve thedesired optical path length. The second ring cavity comprises beamsplitter K11, prisms K12 and K13 and optical plate K14. The second ringcavity doubles the repetition rate a second time using a single looparound the ring cavity. Since the spatial distance between successivepulses is halved after the repetition rate is doubled, the second ringcavity should preferably have substantially half the optical path lengthof the first ring cavity.

The two ring cavities can be fabricated from the same opticalcomponents. Since the two cavities can have substantially similarexternal dimensions, much of the mounting hardware and mechanical designcan be the same. Space can be used more efficiently than in a designwhere one ring cavity has about twice the physical length of the otherring cavity.

FIG. 21 illustrates a ring cavity portion of a repetition ratemultiplier 120L according to an exemplary lens configuration inserted inthe ring cavity formed by prisms L02 and L03 such that the laser beam issubstantially refocused (imaged) back to its original beam condition(waist location and size) when it returns to its original position afterone loop around the ring cavity. Lenses L05 and L06 should preferablyhave equal focal lengths, that focal length chosen so as to form a beamwaist at the center of the cavity, and the lenses are preferably locatedas far away from the waist as practical to avoid the high laser powerdensity near the beam waist. Other components of the laser pulserepetition rate multiplier (such as the beam splitter and optical plate)are omitted from this figure for clarity.

FIG. 22 illustrates another ring cavity portion of a repetition ratemultiplier 120M in which the same lens configuration (i.e., prisms L02and L03 and lenses L05 and L06, see FIG. 21) inserted in a 2-loop cavitywill image the laser beam back to its original beam condition (waistlocation and size) when it returns to its original position. Similarlythis lens configuration will correctly reimage the laser beam for anynumber of loops around the ring cavity.

FIG. 23 illustrates another ring cavity portion of a repetition ratemultiplier 120N utilizing another exemplary lens configuration using twoprisms N02 and N03 and two lenses N05 and N06 arranged such that thelaser beam is imaged back to its original beam condition (waist locationand size) when it returns to its original position. In this embodiment,lenses N05 and N06 should preferably have focal lengths approximatelyequal to the cavity length and be located close to the prisms such thatthe beam waist is formed near the center of cavity and not too close toany of the prism or lens surfaces.

FIG. 24 illustrates another ring cavity portion of a repetition ratemultiplier 120O utilizing another exemplary lens configuration similarto FIG. 23, except that each spherical lens is replaced with a pair ofcylindrical lenses (i.e., spherical lens N05 is replaced by cylindricallenses O051 and O052, and spherical lens N06 is replaced by cylindricallenses O061 and O062. Each cylindrical lens is preferably oriented closeto Brewster's angle relative to the direction of propagation and thepolarization of the laser beam. The advantage of orienting the lenses ator near Brewster's angle for p-polarization is that the reflectivity ofeach lens surface will be very low without using any coating, thussaving the cost of the coating and avoiding any coating damage by thelaser. The curvature of one cylindrical lens in each pair is orientedorthogonally to the curvature of the other cylindrical lens in the samepair (i.e. the curvatures are sagittal and tangential) such thattogether they re-image the beam in a substantially similar manner as oneof the spherical lenses shown in FIG. 23.

FIG. 25A illustrates another exemplary embodiment of a laser pulserepetition rate multiplier 120P with modified right-angle prism pairs(P01 and P02). Prism P01 serves the same function as prism I02 in FIG.17, except that prism P01 is cut and positioned so that the incidenceangle is close to Brewster's angle for p-polarization of the laser beam.Hence no AR coating at the prism is required and potential coatingdamage under high laser power is avoided. Prism P02 has the same shapeas prism P01 and is also oriented close to Brewster's angle relative top-polarization of the laser beam. Prism P02 has an additional coatingonly around point P (not the whole surface), which serve as the beamsplitter that couples the light in and out of the cavity. Thiseliminates the need for a separate beam splitter component, thusreducing the number of optical components and simplifying alignment ofthe ring cavity. As explained above, in one preferred embodiment, thecoating is designed to reflect approximately ⅔ of the energy of eachincident laser pulse so as to produce substantially equal energy in eachoutput laser pulse.

FIG. 25B illustrates another exemplary embodiment of a laser pulserepetition rate multiplier 120Q similar to that of FIG. 25A thatutilizes the same prism P01, but replaces prism P02 of FIG. 25A isreplaced by prism Q02 which has a modified shape. This shape enables theinput and output beams to be parallel to each other, which is generallydesired as it makes it simpler to integrate the laser pulse repetitionrate multiplier into a system. Prism Q02 maintains a right-angle betweenthe TIR faces of the prism and uses an incidence angle close toBrewster's angle for the laser beam in the ring cavity. As describedabove for prism P02, a coating is required around location P in orderfor the prism to perform the beam splitter function.

FIGS. 26A and 26B illustrate a repetition rate multiplier 120R accordingto an exemplary embodiment in which the effective beam path length canbe doubled (or tripled, etc.) for a given physical ring cavity length byrotating one of two right-angle prisms R01 and R02, whetherBrewster-angle cut or not. In these figures, prism R02 having aBrewster-angle cut surface has to be rotated with respect to the axisnormal to this Brewster-angle incidence surface such that the beampolarization state and incidence angle (i.e. the Brewster condition)remain the same after rotation.

FIGS. 27A and 27B are top and side views, respectively, illustrating arepetition rate multiplier 120S according to another multiplier designwith three prisms S01, S02 and S03, each having a different shape, suchthat no coating is needed at any surface. Prism S01 performs the dualfunctions of a reflector in the ring cavity and a beam splitter whichcouples the beam in and out of the cavity at a desired reflectivity,such as a reflectivity of about R=⅓. Note that in the side view shown inFIG. 28B, prism S02 is hidden behind prism S01. Various angles betweensurfaces of the various prisms are indicated in the figures.

The geometry of Prism S01 is shown in FIGS. 28, 28A, 28B and 28C. Itutilizes Fresnel reflectivity properties as shown in FIGS. 29A and 29B.For S-polarized light hitting a surface of fused silica (or any materialwith similar refractive index), the reflectivity naturally goes to about33.3% when angle of incidence is approximately 73°. The input laser beamb1 and the output laser beam b2 are S-polarized with respect to thesurface S1. However, when the refracted beam passes through surface S2or S3, it is P-polarized. If the angle of incidence on these surfaces(S2 and S3) is close to Brewster's angle, the laser beam can pass withminimal loss of power without using any coating and hence avoiding anypossibility of laser damage to the coating.

Prism S02 of repetition rate multiplier 120S (see FIG. 27A) isillustrated in FIGS. 30, 30A and 30B. This prism utilizes reflection atBrewster's angle at surfaces S4 and S5 while using total internalreflection at surface S6.

Prism S03 of repetition rate multiplier 120S (see FIG. 27A) isillustrated in FIGS. 31, 31A 31B and 31C. It serves as a right-angleprism with two surfaces cut at Brewster's angle. These Brewster's anglecuts are oriented for beam polarization perpendicular to the ring cavityplane, which is different from the prism design P01 in FIG. 25A, whichis for polarization parallel to the ring cavity plane.

FIG. 32A illustrates a repetition rate multiplier 120T according toanother exemplary laser pulse repetition rate multiplier configurationwith polarization parallel to the ring cavity plane. This configurationforms a rectangular layout and 90° beam coupling (in/out) which can bemore convenient for incorporation into an instrument. This configurationcomprises three elements. Beam splitter T01 has a coating having achosen reflectivity, such as a reflectivity of R=⅓, at an angle ofincidence of 45° on one surface and having an antireflective (AR)coating on the other surface. As indicated in FIG. 32B, in one preferredembodiment prism T02 is a Pellin Broca prism, which functions as amirror deflecting the beam in a total of 90° with the input and outputsurfaces at Brewster's angle relative to the beam in ring cavity. PrismT02 uses total internal reflection inside the prism. In this way thereis almost no energy loss for a p-polarized beam without using anycoating. In an alternative embodiment, a mirror with a high reflectivitycoating is used in place of prism T02. Element T03 is symbolically drawnas a right-angle prism. In a preferred embodiment, prism T03 uses ageometry with Brewster-cut surfaces such as the design P01 shown in FIG.25A for polarized light with the polarization parallel to the plane ofthe ring cavity.

In a similar manner to that explained above, right-angle prism T03 canbe rotated by 90° about a normal direction to the incident surface, todouble the beam path length within the cavity.

FIG. 33 illustrates a repetition rate multiplier 120U according to anexemplary embodiment in which two ring cavities similar to those of theembodiment of FIG. 32A are cascaded to multiply the repetition rate by4. The first ring cavity U01 has one prism rotated by 90° so that theoptical path length of this cavity is approximately twice that of thesecond cavity U02 even though both cavities have similar physicaldimensions. These two ring cavities together form a multiplier thatquadruples the repetition rate.

In the above cited Ser. No. 13/487,075 application and '940 patent whichare incorporated by reference herein, alternative embodiments of laserpulse repetition rate multipliers are described. These applicationsexplain how the repetition rate of a pulsed laser may be doubled using aring cavity of appropriate length by using a beam splitter to directapproximately ⅔ of the energy of each input pulse into the cavity whiledirecting approximately ⅓ of the energy of each pulse to the output.With a cavity optical path length corresponding to approximately halfthe time interval between the input laser pulses, the output pulsetrains form envelopes of substantially similar energy which repeat at arepetition rate that is twice that of the original laser pulses. The'940 patent also describes how to adjust the transmission andreflectivity of the beam splitter so as to maintain substantially equaloutput pulse energies to compensate for losses in the beam splitter andring cavity. Any of the principles described in the Ser. No. 13/487,075application and '940 patent may be applied as appropriate to the variousembodiments of pulse repetition rate multipliers described herein.

To reach an even higher repetition rate, one can cascade multiple unitsof any of the abovementioned laser pulse repetition rate multiplierswith each unit having a different cavity length. The output repetitionrate can be made equal to 2×, 4×, . . . or 2^(n)× that of the inputrepetition rate where n is the number of laser pulse repetition ratemultiplier cavities, and the optical path length of each cavity is ½, ¼,. . . ½^(n) of the distance between the original pulses.

The above exemplary embodiments illustrate how optical cavities ofdifferent lengths may be formed from various combinations of flatmirrors, curved mirrors, prisms and lenses for the purpose ofmultiplying the repetition rate of a pulsed laser. A repetition ratemultiplier or a repetition rate multiplier that generates an output witha time-averaged substantially flat-top profile may be constructed fromother combinations without departing from the scope of this invention.For example, a flat mirror may be replaced by a prism (or, in manycases, vice versa), or a curved mirror by a combination of a flat mirrorand one, or more, lenses (or vice versa). The choice of which componentsto use is dictated by many practical considerations, including the laserwavelength, the laser power density at the location of the opticalcomponents, the availability of a suitable optical coating for thecomponent, physical space and weight. As explained above, prisms andcomponents with Brewster angle surfaces are generally preferred wherepower densities are high enough to potentially damage optical coatings.

The scope of the invention is limited only by the claims and theinvention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the abovedescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial and derivation that are known in the technical fields relatedto the invention has not been described in detail so that the inventionis not unnecessarily obscured.

The invention claimed is:
 1. A laser pulse repetition rate multiplierfor receiving input laser pulses and for generating output pulses withan output pulse repetition rate that is more than two times that of theinput laser pulses, the laser pulse repetition rate multipliercomprising: a Herriott cell including first and second curved mirrors,first and second beam splitters and at least two fold mirrors forming anoptical cavity, and a right-angled prism positioned outside of theoptical cavity, the Herriot cell being configured such that portions ofeach said input pulse are transmitted along a primary cavity loop insidethe optical cavity, then passed to the right-angled prism, thentransmitted along a secondary cavity loop inside the optical cavitybefore exiting the optical cavity as one of said generated outputpulses, wherein the two curved mirrors have a radius of curvatureapproximately equal to an odd integer multiple of one fourth of aspatial separation between said input laser pulses, and wherein the twocurved mirrors have a common radius of curvature and are separated by adistance substantially equal to the radius of curvature.
 2. The laserpulse repetition rate multiplier of claim 1, wherein the Herriot cell isconfigured such that the output pulse repetition rate is four times thatof the original input pulses.
 3. The laser pulse repetition ratemultiplier of claim 1, wherein the Herriot cell is configured such thatat least a portion of each said input laser pulse is directed by thefirst beam splitter such that it is reflected by first portions of thefirst and second curved mirrors while on the primary cavity loop, and isdirected by the second beam splitter such that it is reflected by secondportions of the first and second curved mirrors while on the secondarycavity loop, wherein said Herriot cell further comprises a prismdisposed in the primary cavity loop, and wherein one of the prism, theat least two fold mirrors, and the right-angle prism is configured todivert laser pulses coming out from the primary cavity loop into thesecondary cavity loop.
 4. The laser pulse repetition rate multiplier ofclaim 3, wherein the Herriot cell is configured such that the primarycavity loop lies in a different plane than the secondary cavity loop. 5.The laser pulse repetition rate multiplier of claim 4, wherein the firstbeam splitter receives said input laser pulses and reflectsapproximately two-thirds of the energy of each said input laser pulseinto the primary cavity loop.
 6. The laser pulse repetition ratemultiplier of claim 1 wherein second beam splitter has a reflectivity ofapproximately one-third.
 7. The laser pulse repetition rate multiplierof claim 1, wherein the Herriot cell is configured such that: each saidinput laser pulse is directed onto the first beam splitter; the firstbeam splitter is configured to direct at least a first portion of saideach input laser pulse such that the first portion is reflected by thefirst and second curved mirrors in a first plane while on the primarycavity loop, and then directed toward said right-angle prism; theright-angle prism is configured to redirect the first portion to thesecond beam splitter; and the second beam splitter is configured todirect at least a second portion of each said first portion such thatthe second portion is reflected by the first and second curved mirrorsin a second plane while on the secondary cavity loop, the second planebeing different from the first plane.
 8. The laser pulse repetition ratemultiplier of claim 1, wherein the Herriot cell further comprises one ofa beam compensator and prism disposed with the first beam splitter in afirst plane, and wherein the second beam splitter and the at least twofold mirrors are disposed in a second plane.
 9. A repetition ratemultiplier for receiving input laser pulses and for generating outputpulses with an output pulse repetition rate that is at least two timesthat of the input laser pulses, the laser pulse repetition ratemultiplier comprising: at least one beam splitter and two lightreflective elements forming a ring cavity, wherein said at least onebeam splitter is configured to direct a first energy fraction of eachinput laser pulse such that the first fraction exits the repetition ratemultiplier at a first time, and configured to direct a second fractionof the energy of the input laser pulse into the ring cavity such thatthe second fraction is reflected between the two reflective elements andexits the repetition rate multiplier at a second time, wherein said twolight reflective elements comprise a first curved mirror and a secondcurved mirror, wherein said at least one beam splitter includes: a firstbeam splitter configured to direct said first and second energyfractions into the ring cavity, and a second beam splitter configured todirect the second energy fraction of each said input laser pulse out ofthe repetition rate multiplier after the second energy fractiontraverses between the first and second reflective elements at leastonce; further comprising: a prism disposed in the ring cavity, at leasttwo fold mirrors disposed in the ring cavity; and a right-angled prismpositioned outside of the ring cavity, wherein the first beam splitterand the prism are configured such that first portions of each said inputpulse are transmitted along a primary cavity loop inside the ringcavity, wherein the second beam splitter and the at least two foldmirrors are configured such that second portions of each said inputpulse are transmitted along a secondary cavity loop inside the ringcavity, and wherein the right-angled prism is configured to direct saidfirst portions leaving the primary cavity loop to the secondary cavityloop.
 10. The laser pulse repetition rate multiplier of claim 9, whereinthe first beam splitter is configured to direct said first portions ofsaid each input laser pulse such that the first portions are reflectedby the first and second curved mirrors in a first plane while on theprimary cavity loop, wherein the right-angle prism is configured todirect the first portions leaving the primary cavity loop to the secondbeam splitter, and wherein the second beam splitter is configured todirect the second portions such that the second portions are reflectedby the first and second curved mirrors in a second plane while on thesecondary cavity loop, the second plane being different from the firstplane.